Technical Field
[0001] The invention concerns magnetic recording tape, the recording layer of which comprises
magnetic iron oxide particles in binder and is made for data recording.
Background Art
[0002] Most magnetic recording tapes that employ magnetic iron oxide particles contain nonmagnetic
particles which are harder than the iron oxide particles. The nonmagnetic particles
both improve the wear resistance of the recording layers and also have a mildly abrasive
action to scrub off any binder material which may be deposited on the recording and
erase heads. Because of that scrubbing action, the nonmagnetic particles are often
called "head cleaning agents", but some publications call them "abrasive powder" or
"reinforcing agents" or simply "hard powder." In data recording tapes, the most commonly
used head cleaning agent, alumina (A1203), is typically used in amounts from 2 to
10% by weight of the magnetic iron oxide particles.
[0003] Most data recording mechanisms employ ferrite heads which are quite resistant to
abrasion. Even though magnetic recording tapes are drawn across the heads at high
speeds up to about 500 cm per second, alumina particles in amounts within the above-mentioned
range do not unduly abrade ferrite heads. That is, a ferrite head normally fails for
other reasons before its gap is worn away by the abrasive action of the alumina. For
example, a ferrite head may fail from work-hardening degradation or from gap erosion.
[0004] Far more economical than ferrite heads for data recording are soft heads, such as
brass and "Mumetal" heads, but they would quickly be destroyed by conventional data
recording tapes containing effective amounts of alumina. Accordingly, manufacturers
of data recording mechanisms that employ soft heads suggest the use of data recording
tapes that are less abrasive than those containing hard, nonmagnetic particles such
as alumina and that instead users should be more conscientious about periodically
cleaning the heads with a solvent that is specially designed to remove deposited binder.
This puts an extra burden on users, and any lapse in periodic cleaning can result
in lost or erroneous data.
[0005] Hence, there continues to be a need for a data recording tape that has good resistance
to wear and will keep soft heads reasonably clean without undue wear.
[0006] Except as noted above, we have found nothing in the prior art about the problem of
preventing undue wear of soft data recording heads while also minimizing tape wear.
On the other hand, we are aware of a great deal of prior art dealing with both head
wear and tape wear in video recording. For example, United States Patent No. 4,275,115
(
Naruse) says that the addition of "hard powder to a magnetic recording composition
... fails to improve the durability against the still picture reproduction and correct
the tendency to the abrasion loss" (col. 1, lines 33-36). The Naruse patent's answer
is to employ at least two types of nonmagnetic powders, one having a Mohs' hardness
greater than 7 and the other a Mohs' hardness smaller than 7 but greater than about
3. Among powders listed as having a Mohs' hardness greater than 7 are "Cr
20
3, alumina, silicon carbide, quartz, titanium oxide, zirconia and the like" (col. 2,
lines 7-9).
[0007] United States Patent No. 4,420,408 (Kajimoto et al.) also concerns video tape and
the problem of improving its wear resistance to improve the still characteristic.
The Kajimoto patent solves this problem by combining two kinds of fine "abrasive powders",
both having a Mohs' hardness of at least 7, but having different true specific gravities
differing by more than 1.0. Table 1 reports still reproducing times when employing
equal parts of several pairs of powders, one pair being Ti0
2 (true specific gravity 4.2) and Cr
2O
3 (true specific gravity 5.2).
[0008] European Patent Application 108,971 says that to enhance the still picture reproducing
characteristics of video tape "it has heretofore been proposed to incorporate as a
reinforcing agent a nonmagnetic powder material such as chromia (Cr203), alumina (Al
20
3), or silicon carbide (SiC) into the magnetic layer. It is not easy, however, to disperse
uniformly the above reinforcing agents in the magnetic layer, and there occurs non-uniformity
of the dispersion, resulting in a decrease in wear resistance of the magnetic layer,
the excessive wearing of the magnetic head, or a decrease in smoothness of the tape
surface" (page 2, lines 5-15). The application is directed to a way of better dispersing
the reinforcing agents which it also calls "abrasives", "nonmagnetic powder material",
and "inorganic powder material". Useful inorganic powder materials are listed at page
4, lines 15-19 and include titanium dioxide (Ti0
2) which also is used in Example 8 and 13.
[0009] U.S. Patent No. 4,399,189 (Nakashima) concerns "two inconsistent requirements of
an improvement of the wearing resistance of the magnetic tape for improving still
characteristic and a reduction of the wearing of the head of the recorder" (col. 1,
lines 32-39). It says that while Cr
2O
3 and Al
2O
3 powders improve the wearing resistance of magnetic recording tape, a wearing of the
head is disadvantageously increased. Its solution to this problem is to employ in
the magnetic layer both "a first non-magnetic powder of Al203 and/or Cr
20
3 at a ratio of 0.5 to 15 weight % based on the magnetic powder and a second non-magnetic
powder of TiO
2, SiC, Si0
2, ZrO
2 and/or Ce0
2 at a ratio of 0.5 to 15 wt. % based on the magnetic powder and a total of the non-magnetic
powder of less than 20 wt. % based on the magnetic powder. The average particle diameter
of the first and second non-magnetic powder is in a range of 0.1 to 2 micrometers"
(col. 1, lines 46-57). Although the patent has no working example, it gives the general
formulation of tapes including both Ti0
2 and Al203 powders at various ratios. Then in commenting on drawings generated from
those tapes, it says: "When a ratio of A1
20
3 powder was more than 3%, the still reproducing time was increased to be about 220
minutes. Moreover, if a ratio of
Ti
O2 powder was higher in said condition, the still reproducing time was further increased"
(col. 3, lines 38-42).
[0010] United States Patent No. 3,929,658 (Beske) concerns magnetic recording tapes using
as the magnetic material CrO
2 which is quite hard and abrasive. Under "Background of the Invention", the Beske
patent points out that magnetic recording tapes cause undue head wear and mentions
two prior approaches to improving head life. One approach uses lubricants and the
other uses "very hard particles that are even more abrasive than the ferromagnetic
particles, such as flint, garnet, silicon carbide, as described, for example, in U.S.
Pat. No. 3,630,910 and in German Pat. Nos. 1,804,393 and 1,953,459" (col. 1, lines
46-50). The Beske patent instead improves head life by employing "about 2 to about
20 percent, based on the weight of the ferromagnetic particles, of finely divided
particles of at least one nonferromagnetic metal oxide having a hardness of 5.5 or
less on the Moh scale" (col. 1, lines 59-63). "Particularly preferred materials are
anatase titanium dioxide (Moh hardness = 5.5) and molybdenum trioxide (Moh hardness
= 2.0)" (col. 3, lines 33-36). While the patent does not say so, it appears that the
titanium dioxide and other nonferromagnetic particles are being used to offset the
overly abrasive nature of the ferromagnetic Cr0
2.
[0011] Although none of the above-discussed publications specifically concerns data recording
or problems arising out of the use of soft heads, each may have some pertinence to
the present invention because of the use of titanium dioxide in the magnetizable layer.
[0012] Because Mohs' hardness values are rather indefinite, hardness values are here reported
in the Knoops scale. With either scale, the hardness of a powder can only be estimated
from the hardness of a crystal of the same material, and there can be no assurance
that the hardness remains the same after the crystal has been ground into a fine powder.
Furthermore, various forms of the same chemical compound may have significantly different
hardnesses. In the above citations, the Mohs' hardness of TiO
2 is given both as 7.0 (Naruse and Kajimoto) and as 5.5 (Beske). The CRC Handbook of
Chemistry and Physics, 56th addition, at pages B-197 to B-200, reports Mohs' hardnesses
for various crystalline forms of TiO
2 to be: anatase 5.5-6.0, brookite 5.5-6.0, and rutile 6.0-6.5. We have made no effort
to measure hardnesses and simply repeat reported Knoops hardnesses for a number of
nonmagnetic materials as follows:
[0013] All of the magnetic recording tapes we have studied use one of the first two of this
list as head cleaning agents, and SiC reportedly is sometimes used. Because of their
hardness, we here call them "hard head cleaning agents" and use that term to indicate
a nonmagnetic powder having a Knoops hardness greater than 1200.
[0014] The CRC Handbook at page F-22 compares Knoops and Mohs' hardness values for various
materials.
Disclosure of the Invention
[0015] The invention provides magnetic recording tapes having a magnetizable layer comprising
ferromagnetic iron oxide particles dispersed in binder, which tape can be used on
data recording mechanisms that have soft heads and can keep those heads reasonably
clean without undue wear. The novel magnetic recording tape achieves this by including
in its recording layer, based on the weight of the iron oxide particle, from 0.5 to
3% of titanium dioxide or other nonmagnetic powder having a Knoops hardness from 600
to 1200 (preferably less than 800) while restricting the amount of any hard head cleaning
agent to less than 1.0%. We here use the term "soft head cleaning agent" for nonmagnetic
powder having a knoop hardness from 600 to 1200.
[0016] It is surprising that titanium dioxide could have this extraordinary effect, considering
that its hardness may approximate or only slightly exceed that of ferromagnetic iron
oxide particles. Although no report of Knoops hardness values for ferromagnetic iron
oxides has been found, the aforementioned CRC Handbook reports Mohs' hardness of 5-6
for Fe
20
3 (hematite) and 5.5-6.5 for Fe
30
4 (magnetite).
[0017] Even though soft head cleaning agents such as titanium dioxide have been used in
magnetizable layers for reasons such as we discussed in citations of the above "Background
Art" section, it is our understanding that they have been used only to modify the
abrasivity of other ingredients, e.g., the abrasivity of Al
20
3 or CrO
2. We do not find in those citations any suggestion of using as head cleaning agents
titanium dioxide (or other powders we here call "soft head cleaning agents").
[0018] Preferably, the amount of soft head cleaning agent does not exceed 1.5%, because
larger amounts have shown no advantage and would limit the amount of the ferromagnetic
iron oxide particles, hence limiting signal output. On the other hand, it occasionally
may be desirable to limit signal output.
[0019] In the magnetizable layer of any magnetic recording tape of the invention, the soft
head cleaning agent and any hard head cleaning agent should be substantially free
from particles exceeding 2 micrometers in diameter. Larger particles could scratch
the heads, and also create head-to-tape spacing losses. It is preferred that the average
diameter of the soft head cleaning agent be from 0.2 to 1.0 micrometer and that of
any hard head cleaning agent be from 0.2 to 0.5 micrometer. Best results to date have
been achieved when the amount of soft head cleaning agent exceeds that of any hard
head cleaning agent. Preferably, the recording layer is free from hard head cleaning
agent.
[0020] In the following examples, all parts are by weight.
Example 1
[0021] The backing of this tape of this example was biaxially oriented poly(ethylene terephthalate)
film having a thickness of about 1.0 micrometer. A nonmagnetic coating had been applied
to the backside of the film to provide a degree of roughness and electrical conductivity,
after which a magnetizable layer was applied to the faceside using the following dispersions.
[0022] Dispersion I was milled for 24 hours in a jar mill with ceramic media. Dispersion
II was made by milling until smooth in a vertical sandmill and then mixed with a high
speed mixer to which sufficient Dispersion I was added so that the final mixture had
1.5% titanium dioxide based on the iron oxide particles. Immediately after adding
isocyanate crosslinking agent, the combined dispersion was coated onto the backing
and heated to drive off the solvent, providing a dry-coating caliper of about 2
[0023] micrometers. This was then calendered, yielding a final magnetizable coating caliper
of about 1.75 micrometers. The resulting tape was slit to 1/4 inch (0.635 cm), and
183 m was loaded into a belt-driven data cartridge of the type disclosed in U.S. Patent
No. 3,692,255.
Example 2 and Comparative Examples A, B and C
[0024] Tapes were made and loaded into cartridges as in Example 1 except substituting for
the titanium dioxide the same amount of other nonmagnetic powders as follows:
Each cartridge was driven in a DCD-3 drive (Minnesota Mining and Manufacturing Company)
across a brass head at a speed of 90 inches per second (229 cm/sec). After 2500 cycles,
measurements were made of the physical wear profile of the brass head. Also, the head
was examined visually for deposited debris. A rating from 1 to 10 was awarded, 1 indicating
substantially no deposit and 10 indicating a large deposit. Results were:
[0025] A deposit rating of 4 or less is considered to be satisfactory, a rating from 5 or
6 is considered marginal, and a rating of 7 or higher is considered to be unsatisfactory.
The fact that comparison tapes
B and C left greater deposit than those of Examples 1 and 2 is attributed to debris
generated in the excessive wearing of the head.
Examples 3 - 6 and
Comparative Tapes D - H
[0026] A series of tapes were made substantially as in Example 1 except employing the following
dispersions and various amounts of the titanium dioxide.
[0027] For comparison, a series of tapes were made using A1
20
3 powder (average diameter 0.5 micrometer) instead of the titanium dioxide.
[0028] Each of the tapes was loaded into a belt-driven data cartridge and tested as above.
Examples 7 - 9 and Comparative Tapes I - K
[0029] A series of tapes were made substantially as in Examples 3-6 and Comparative Tapes
D-H except employing the following iron oxide dispersion:
The tapes were loaded into cartridges and tested as above.
[0030] The examples demonstrate that by substituting titanium dioxide powder for a conventional
head cleaning agent, a data recording tape keeps a brass or other soft head reasonably
clean without undue head wear.
1. Magnetic recording tape suitable for data recording mechanisms equipped with soft
heads, said tape having a magnetizable layer comprising ferromagnetic iron oxide particles
dispersed in binder, characterized in that said layer includes, based on the weight
of the iron oxide particles, from 0.5 to 3% of soft head cleaning agent while restricting
the amount of any hard head cleaning agent to less than 1%, and the soft head cleaning
agent and any hard head cleaning agent are substantially free from particles exceeding
2 micrometers in diameter.
2. Magnetic recording tape as defined in claim 1 further characterized in that the
Knoops hardness of the soft head cleaning agent is less than 800.
3. Magnetic recording tape as defined in claim 2 further characterized in that the
soft head cleaning agent is titanium dioxide.
4. Magnetic recording tape as defined as claim 3 further characterized in that the
amount of titanium dioxide exceeds that of any hard head cleaning agent.
5. Magnetic recording tape as defined in claim 4 further characterized in that the
recording layer is free from hard head cleaning agent.
6. A data recording mechanism equipped with a soft head and a magnetic recording tape
as defined in claim 2.
7. A belt-driven data cartridge loaded with a magnetic recording tape as defined in
claim 2.
8. Method of making the magnetic recording tape of claim 1 said method comprising
the steps of dispersing into a binder ferromagnetic iron oxide particles and characterized
in that from 0.5 to 3% by weight of soft head cleaning agent are dispersed into said
binder while restricting the amount of any hard head cleaning agent to less than 1.0%
based on the weight of the iron oxide particles, and coating the dispersion onto a
nonmagentic backing.
9. Method as defined in claim 8 further characterized in that the Knoops hardness
of the soft head cleaning agent is less than 800.
10. Method as defined in claim 9 further characterized in that the soft head cleaning
agent is titanium dioxide.
11. Method as defined in claim 10 further characterized in that the amount of titanium
dioxide exceeds the amount of any hard head cleaning agent.